I have a 31.6mm carbon-wrapped aluminum seatpost which fit perfectly fine in my previous aluminum frame. Unfortunately, that frame cracked, but I found the exact make, model, and size frame on eBay to replace it. The only problem is that the seatpost doesn't fit into the seat tube very well. It is a very tight fit, and I can only get it down 1 1/4" before it gets really difficult, even with a bit of grease.

The top of the seat tube doesn't appear to be crimped, so I'm not sure what the problem is. I'm a bit gun-shy since I've already broke a frame this year. So, I'm not sure whether to force it down, use a rubber mallet, or buy a new seatpost.

How do I determine if it's possible to fit this seatpost in this frame safely? Is it possible there's something else interfering with the installation, or will I need to buy a new seatpost?

Pull the post out and check the inside of the tube for a blob of paint or some such blocking it.
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Daniel R HicksFeb 13 '12 at 1:15

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Welcome to the site! As you may already know, Stack Exchange sites prefer questions that are specific and answerable. The question here was pretty obvious, so I've taken the liberty of turning your request for suggestions into a specific question. In the event that I'm mistake here, please feel free to revert my edit.
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Neil FeinFeb 13 '12 at 1:54

The inside of the seat tube is clear. All that is visible is the line where the previous seatpost sat, dust, and the freshly greased area that my current seatpost made it to.
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SF RoadieFeb 13 '12 at 5:38

There's always the possibility that the new frame is not really identical to the old one. For some reason seatposts come in a bizarre array of sizes, so the frames could differ by design by 0.2 or so.
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Daniel R HicksFeb 13 '12 at 13:16

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If they are the same make, model, and year frame and the proper sized seatpost will not insert into the frame more than 1-1/4" then something is wrong.This one's hard to diagnose over the internet so I suggest you head down to a reputable bike shop and let them have a look at the frame before something gets mucked up irreparably.

It is the same model and year, but it is Italian and my LBS mentioned that sometimes they grab what's available. Apparently, my newer frame has a 31mm seat tube, whereas the previous one had a 31.6mm. I'm not sure which one is the proper size, but it looks like I need a new seat post. Thanks for all the help!
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SF RoadieFeb 16 '12 at 2:32

In order to check the inside of the frame, visual inspection most times FAIL, because the problem is caused by tiny bumps or misalignments that are not easily seen with naked eye and even felt with fingertips.

I would recommend that you take another seatpost (an old one you can take/borrow on a local bike shop) and try to insert more aggressively. You should polish the seatpost first someway (steel wool, for example).

You force it inside a bit, then take it out. Probably, there would be some marks on the seatpost surface, suggesting if there is some irregularity inside the frame. Most common are bending around the vertical cut around the clamping nut, or deformation caused by welding the tubes around that region too deep (to the point of almost melting the inside of the tube.

The seatpost tube of the frame might be not-straight, which would be a manufacturing defect.

In the end, you could polish/sand/grind inside the frame to make the seatpost go down easier, but since seatposts that don't go down so easy come out very hard afterwards, and your post has a carbon surface, taking the frame to the bike shop to check if it needs replacement or special service (with machining tools to improve surface quality and alignment) could be necessary.